Guide number
From The Glossary of Digital Photography
A numerical rating for the power of an electronic flash. By convention, the guide number is set for ISO 100. It is calculated by multiplying the f-stop by the distance for a proper exposure of the subject at the maximum distance from the flash. Thus, if a subject at 15 feet were properly exposed at f /4, then the guide number would be 15 x 4, or 60. Some manufacturers have been known to be 'generous' when describing the guide number of their flashes. Be sure to test a flash or rely on an independent source before accepting a guide number. Note that the conditions of the test, such as a small room, a room with light-colored walls, diffusion attachments, or zoom settings on a flash, will significantly affect the results.
